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With the rapid advancement in the smelting capabilities of domestic steel mills, the performance of domestically produced low-temperature quenched and tempered high-strength steels has reached international first-class standards. In recent years, more and more spherical tanks made of quenched and tempered high-strength steel have been adopted in order to reduce the steel plate thickness and welding volume of large-capacity, high-pressure spherical tanks. This article revises some of the statements from the previous one and adds information regarding ultrasonic testing of steel plates, for reference. The several steel grade numbers of low-temperature quenched and tempered high-strength steel that are commonly used in spherical tanks include Q490DRL2 (-50°C) and Q490DRL1 (-40°C) ； Commonly used low-alloy steel grades include Q345R and Q370R ； The commonly used low-temperature carbon-manganese steels are mainly 16MnDR and 15MnNiNbDR. Here is a brief introduction to the main differences among the above three types of steels used for spherical tanks. 1. Design method: Considering that the unit price of low-temperature quenched and tempered high-strength steel is generally about 60% higher than that of ordinary alloy steel, when the thickness of the spherical tank exceeds 30 mm, a stress analysis design method is usually adopted for such tanks made of low-temperature quenched and tempered high-strength steel. This not only helps to reduce steel consumption but also enables more accurate and comprehensive analysis and calculation of overall and local stresses, allowing various types of stresses to be kept at reasonable levels and ensuring the safety of the spherical tank. For spherical tanks made of ordinary carbon-manganese alloy steel (including low-temperature carbon-manganese steel), design is still primarily carried out in accordance with standards, resulting in a relatively low threshold for such design. 2. Delivery condition of the steel plate: For Q345R steel plates with a thickness of 36 mm or less, they can be delivered in either hot-rolled or controlled-rolling condition; for plates with a thickness of 36 mm or more, they must be supplied in normalized condition. For Q370R steel plates, all thicknesses must be supplied in the normalized condition. When the medium is highly hazardous or prone to stress corrosion, even if it is thin, Q345R steel plates should be specified to be supplied in a normalized condition during design. Low-temperature carbon-manganese steel must be supplied in the normalized condition; when the thickness is large or special requirements exist, it should also be supplied in the normalized + tempered condition. The supply condition of low-temperature quenched and tempered high-strength steel plates is quenched and tempered, that is, they undergo high-temperature tempering after quenching. Quenching and tempering heat treatment places higher demands on the heat treatment furnaces in steel mills; therefore, only large-scale steel mills generally possess the capability to carry out such treatments effectively. Special note: To ensure that post-weld heat treatment does not damage the properties of the quenched and tempered steel plate, and to prevent it from weakening the mechanical properties of the material, it is essential to carefully consider the relationship between the tempering temperature of the steel plate and the maximum temperature during post-weld heat treatment, ensuring that the latter is not higher than the high-temperature tempering temperature of the steel plate. 3. The acceptance criteria for the mechanical properties of steel plates differ: Low-temperature quenched and tempered high-strength steel plates require not only low-temperature impact tests at lower temperatures, but also a drop-weight test, as well as tests on the mechanical properties of the core portion of the steel plate. Moreover, the ultrasonic testing requirements for low-temperature quenched and tempered high-strength steel plates are generally at level T1, which is higher than the level I or II requirements for ordinary steel plates. Ordinary low-alloy steel plates generally only require routine mechanical property tests in accordance with the standards for such plates. 4. Similarities and differences in the ultrasonic testing requirements for steel plates: In the case of ordinary media with a thickness of no more than 36 mm, Q345R needs to meet at least grade III. For 16MnDR and 15MnNiNbDR, if the thickness is greater than 20 mm, just like in the case of low-temperature high-strength steels, the ultrasonic inspection level must be at least grade I to be considered satisfactory. For details, please refer to the relevant provisions in the table below. 5. The requirements during the manufacturing process are also more stringent: Low-temperature quenched and tempered high-strength steel spherical tanks demand higher precision in the fabrication of the spherical shell plates, as well as stricter requirements for weld grinding. Moreover, the control of temperature uniformity during heat treatment is more rigorous, with it being necessary to ensure that the heat treatment temperature does not exceed the tempering temperature. 6. The requirements for non-destructive testing are also more stringent: Higher-grade low-temperature quenched and tempered high-strength steels generally entail higher requirements for non-destructive testing; for example, the technical level for TOFD testing is raised to grade C ； Or raise the qualified level from Grade II to Grade I ； Moreover, low-temperature quenched and tempered high-strength steel spherical tanks require 100% magnetic particle testing both before and after the hydrostatic test. The table below shows the requirements for the minimum acceptable level of various non-destructive testing methods. 7. Some other special requirements: Due to their high yield strength ratio, low-temperature quenched and tempered high-strength steels require a higher level of purity in the steel material. For example, impurities such as sulfur and phosphorus in the chemical composition need to be controlled at levels lower than the standard requirements, and control limits also need to be set for non-metallic inclusions. The others also involve the control of carbon equivalent and hardness parameters ； Test requirements for the simulated post-weld heat treatment of quenched and tempered steel plates and associated forgings, etc. 8. Welding material selection: At present, for low-temperature quenched and tempered steel spherical tanks in China, about half still choose imported welding materials from abroad. Given the high prices and long lead times of imported welding materials abroad, many owners are willing to opt for domestic alternatives. In China, several top-tier suppliers of high-strength steel welding materials already possess a high level of technical expertise and extensive practical experience; in fact, some of their product specifications show better performance compared to imported welding materials. 9. Assembly and welding: The accuracy of assembly has a direct impact on the quality of welding that follows. If the pressing accuracy is poor, and quality control is not strict during installation, cracks are likely to occur in low-temperature high-strength steel after welding. The construction of low-temperature high-strength steel spherical tanks requires excellent pressing techniques for such steel plates, extensive assembly experience, and a highly skilled welding workforce.